1. Field of the Invention
The instant invention is directed to a process for the preparation of stretched polymeric film. More particularly, the instant invention is directed to a process for preparing stretched polymeric films in which more than two piles of the film are stretched simultaneously.
2. Background of the Prior Art
An important processing step in the preparation of many polymeric film products is one or more stretching steps. Stretching, which may be uniaxial or biaxial, is employed to impart one or more characteristics or properties to the film. For instance, stretching may be employed to orient the film to provide improved tensile characteristics. Other films are stretched to change them from a non-open-celled film to an open-celled microporous film. Still other films are stretched for purposes that are specific to their application. Suffice it to say, a significant percentage of commercial film production includes the processing step of stretching formed film in one or more steps, and in one or more directions.
Polymeric film stretching requires that the film be in contact with machinery which stretches the film. For instance, a typical means for stretching film is by contacting the film between a first pair of rollers and thereafter by a second pair of rollers, disposed downstream of the first pair of rollers. The second pair of rollers is rotated at a speed in excess of the first pair of rollers. The excess speed imparted to the film by the second pair of downstream rollers results in stretching of the film between the two sets of rollers. In the course of such a procedure the surface of the film may suffer surface damage of imperfections due to the contact of the heavy rolls on the polymeric film.
Other means for stretching film also require contact between heavy machinery and film. Typically, film is stretched in the transverse direction by means of a tender. Tenters are machines which hold the film at its sides and extend outward to increase the width of the film. Specifically, the size of the film in contact with the tenter is usually slit and discarded.
The surface damage which may be imparted to stretched films have serious deleterious effects on certain important properties of polymeric film. One such effect is the optical properties of the film. Obviously, a film surface that is damaged, scratched or the like does not provide the type of surface necessary to provide the optical properties necessary in certain film applications. Metallized films, for one, require a perfectly clear surface. Packaging films which include printing on their surface also require good optical properties to provide a suitable substrate for the printed material.
Freedon from surface defects is a critical property in open-celled films such as porous and microporous films. Surface defects have an appreciable effect on the gas transmittability property of these films. The gas flux, a very important property in an open-celled film, is usually decreased when subjected to surface defects due to the closing of pores. In addition, the stability of open-celled films, as measured by variation in gas flux with time, is similarly adversely affected by surface defects.
Another important defect which arises during conventional film stretching operations resides in the exposure of the film to surface contamination. This is especially significant in such applications as microporous film which are often employed in medical applications, i.e., burn dressings and the like.
In conventional single or double ply stretching, at least one surface of each stretched ply is exposed to the atmospheric conditions in the area of the stretching apparatus. As those skilled in the art are aware, unless expensive precautions are observed, these conditions can result in surface contamination.
In addition to the deficiencies of the prior art represented by the inability of the processes of the prior art to produce a stretched film totally free of surface defects, film processing involving a stretching operation has an inherent production limitation limiting the rate of the formation of the stretched film. This limitation applies to all films, not only those whose surface properties are of critical importance. The prior art does not suggest any method by which the processing speed of stretched films may be improved.
For the above reasons a new process providing a new means for stretching film, which does not result in surface damage to the film, and which increases the production rate would represent a significant advance in the film forming art.